It is well-known that quite a lot of raw materials as e.g. phosphors, required in order to build up intensifying or storage phosphor plates, have a fair to high sensitivity for moisture, i.e. they are hygroscopic to some extent and tend to absorb water. Absorption of even small amounts of water rapidly reduces the efficiency of the phosphors to vanishingly small values.
Most of those plates or panels coated with phosphors containing halide suffer from this draw-back. Hygroscopic phosphors used in intensifying screens are e.g. BaFCl:Eu, BaFBr:Eu, LaOBr:Tm and GdOBr:Tm. Examples of hygroscopic X-ray storage phosphor that can be used in computer radiography systems are: BaFBr:Eu, BaFI:Eu, LaOBr, (Ba,Sr)F(Br,I):Eu, RbBr:Tl, CsBr:Eu, CsCl:Eu and RbBr:Eu. In order to make these hygroscopic phosphors suitable for use it is necessary to protect the phosphor particles from moisture.
One way to protect the individual phosphor particles is by coating them with a substance such as e.g. SiO2 or TiO2 which prevents penetration of moisture as has been disclosed in EP-A's 0 097 377, 0 476 206 and 0 928 826; in GB-A 2,255,100; in U.S. Pat. Nos. 5,196,229 and 6,177,030 and in WO 00/22065. In the alternative said phosphor particles are coated with a UV-absorbing transparent synthetic resin to make them resistant to moisture, oxidation and ultraviolet irradiation as has been disclosed in JP-A 60-177090.
However, it is very difficult to apply a defect-free coating to particles having an average particle size of 1 to 10 μm. Moreover if the coating on the phosphor particles is too thick the amount of phosphor in a layer should be reduced in order to avoid thickening of the coated phosphor layer. As this leads to reduced X-ray absorption by the phosphor screen or panel no acceptable solution can be expected from that measure so that applying a coating around the individual phosphor particles is, therefore, not always desirable.
Another way to stabilize a phosphor screen containing a hygroscopic phosphor is by adding a stabilising compound to the phosphor layer as has been disclosed e.g. in EP-A's 0 234 385, 0 506 585, 0 544 921 and 0 747 908 and in U.S. Pat. Nos. 5,639,400 and 5,641,967. This compound will either react with the absorbed water, thereby preventing reaction of the phosphor particles with water, or it will react with the breakdown products which are formed by reaction of the phosphor particles with moisture, thereby preventing discoloration of the screen. Discoloration is well-known as a common cause of deterioration of screen efficiency. It is clear that stabilising substances thus have a finite working period. Since water is attracted continuously the amount of unreacted stabilising compound diminishes with time. After a certain period of time, no unreacted stabilising material will be left and the phosphor screen will undergo delayed deterioration.
Stabilising compounds are known as additives to powder phosphor screens, in which the active layer is produced by coating a lacquer consisting of phosphor particles, a binder and a solvent to a substrate. Some phosphor screens are produced by means of physical vapor deposition in a vacuum chamber, although it is not obvious to introduce a stabilising compound into a vapor deposited phosphor layer. Yellowing of phosphors having iodine can be prevented by diverse stabilising compounds as has been disclosed e.g. in EP-A 0 234 385, in U.S. Pat. No. 5,641,967 and in JP-A 62-247300 63-193096, 63-193097, 63-193098 and 63-193099 and in JP-B 94-031909.
A better way in order to protect a phosphor screen is related with the application of a protective coating or coatings as has been described e.g. in EP-A's 0 209 358, 0 348 172, 0 654 794 and in U.S. Pat. No. 4,603,253, wherein said coatings have been applied on top of the active layer containing the phosphor particles, also called “phosphor layer”. Protective layers may be applied by several techniques. A less preferred way is to coat a polymer solution onto the phosphor layer and to subsequently evaporate the solvent by heating. A disadvantage of this technique namely is that the solvent my cause swelling and damaging of the phosphor layer. A more preferred way is to laminate a film onto the phosphor layer. Another preferred way is to apply a monomer lacquer onto the phosphor layer by e.g. screen printing and to cross-link the monomers by ultraviolet or electron beam irradiation, thus forming a continuous topcoat layer free of defects as has been described e.g. in U.S. Pat. Nos. 5,520,965 and 5,607,774.
All techniques mentioned thus far however suffer from the draw-back that the edges of the phosphor layer remain unprotected. Hence water may penetrate the phosphor layer from the side and degradation and discoloration may start from the edges, thereby moving inwards further.
An edge protection layer may further be applied separately as has been disclosed e.g. in EP-A's 0 095 188 and 0 576 054, as well as in JP-A 63-193098 and in JP-B 94-31914. This requires-an additional step in the manufacturing process. In addition, adhesion between the protective layer and the edge enhancing layer is not perfect, which makes the edge enhancement layer vulnerable. Upon use of the screen, the edge enhancement layer is damaged and screen degradation at the edges can proceed locally again.
Intensive use of screens or panels showing no loss of image quality is required over a period of 3 years, and, more preferably, over a period of 5 years, during which about at least 10,000 cycles should have been run with one and same screen.
A solution has been found in EP-A's 1 286 362 and 1 286 365 in order to overcome the problems as set forth, more particularly with respect to moisture sensitivity, by applying a protective layer over the phosphor layer by the method of chemical vapor deposition (further in the text indicated as “CVD”) in order to uniformly cover surface of the panel, so that the screen can be completely enveloped in a protective coating that extends seemlessly from the top of the screen, over the edges onto the back of the screen, thereby preventing moisture penetration, both at the top and at the edges. In a preferred embodiment thereof the phosphor screen in said EP-A's is a needle storage phosphor screen, composed of a CsX:Eu2+ storage phosphor, X being selected from the group of halides consisting of Br and Cl, and the moisture proof protective layer is a “parylene” layer, “parylene” being a generic name for thermoplastic polymers and copolymers based on p-xylylene and substituted p-xylylene monomers.
However the problem remains stringent, depending upon local climate environment, when using and storing the panels over a long period of time as moisture and oxygen are invasive compounds, able to deteriorate the excellent properties of the storage panels on the long term as set forth hereinbefore.